During a flight, the pilot of an aeroplane generally uses a meteorological observation system deriving from a radar that gives him information concerning the state of the atmosphere in order to detect the risks inherent in the meteorological situations making it possible among other things to restore a Doppler speed field and a radar reflectivity field in three dimensions, 3D. These fields are obtained by performing successive scans with the radar according to different bearing and elevation angles. At each point of a 3D grid covering the extent of observation, radar reflectivity information is thus in particular made available that is dependent on the wavelength used and the reflection, absorption and diffusion properties of the targets present in the radar resolution volume. On the control screen, in the cockpit, the pilot then sees meteorological information displayed that is generally analysed on the basis of four colour levels corresponding either to the absence of signal or to a weak, moderate or strong reflectivity factor. The pilot interprets this information in terms of risk to decide if he can maintain the trajectory planned for his aeroplane or if he must modify it to avoid an area exhibiting meteorological risks, for example an area where a cloud is forming.
To simplify the approach of the pilot in his decision-taking, it is useful to translate the meteorological fields observed by the radar into a field describing the level of meteorological risk.
Generally, the meteorological radars on board aeroplanes operate in the X band. The radar reflectivity measured by a meteorological radar in the X band using a single wavelength exhibits an ambiguity regarding its relationship with the nature and the characteristics of the hydrometeors, such as water droplets, snowflakes or hail notably, that have generated it. In practice, this relationship is strongly non-bijective. Consequently, one and the same reflectivity level can be generated by hydrometeors of very different kinds, originating either from snow or hail notably, corresponding to meteorological situations that are also very different from the point of view of the aeronautical risk, for example stratiform clouds and thick convection clouds such as cumulonimbus. This bijectivity defect notably concerns the hail consisting of hailstones of a size greater than approximately a centimeter of equivalent spherical diameter. These hailstones fall outside the Rayleigh scattering domain, in which the meteorological radars usually work. They then fall in the Mie scattering domain which is such that when the size of the hailstone increases, its reflectivity diminishes.
It is therefore necessary to remove this ambiguity and obtain with a meteorological radar both information concerning the dynamic nature of the clouds generating the reflectivity field, stratiform clouds or intense convective clouds of stormy cumulonimbus type, and concerning the microphysical nature of the hydrometeors, deriving from rain, snow or large hailstones.
Solutions are known for eliminating this ambiguity. In particular, it is known how to determine the level of risk at each grid point by using simple thresholds on the radar reflectivity factor observed at these points. If the latter is greater than 40 dBZ, the risk is considered to be high whereas, if it is less than 30 dBZ, the risk is considered to be low. Between these two values, the risk is considered to be moderate. These thresholds can, if necessary, differ according to the regions being flown over in accordance notably with the description in the U.S. Pat. No. 7,129,885. This method of estimating the risk solely by the one-off value of the radar reflectivity notably has the drawback that it can lead to an underestimation or an overestimation of the risk in certain meteorological situations. For example, a strong snow shower in a stratiform system can be seen by a radar with high reflectivity factors, greater than 40 dBZ, although the situation presents no risk to the aircraft in flight.